Comparison: Steel, Concrete, and Steel-Concrete designs
In the civil engineering community, there are three major types for designing the structures based on the needs and specified features. However, the three major types of design are: steel design, concrete design, and steel-concrete design. The main differences between theses three types are: the definition, the types of forces and loads, the followed codes, the stability of the structure, uses, and some environmental issues.
Definition. The steel design method means that the percentage of steel in the structure is about 70% including the dry walls and the insulation systems. By contrast with the concrete design, the structure can be completely constructed by concrete with only 20% of steel bars and ties for columns and beams. However, the steel-concrete design combines the last two types to reinforce the structure by 50% of steel and 50% of concrete. In fact, the last type is the most preferable for many civil structural engineers around the world.
Types of forces and loads. As a result of difference in used material in the structure, the types of forces and loads will also differ. In the steel design the horizontal loads like winds or earthquake is considered; on the other hand, the tension forces are not an important issues in the steel design because of the high elasticity of the used steel bars. In contrast with the concrete the tension is the problem not the horizontal forces or even the compression. However, the steel- concrete design comes over the defects of the last two types. The reinforced steel bars will take the tension; on the other side, the concrete will take the compression and the horizontal forces.
Followed Codes. Commonly, the followed code for steel design is the standard American code for steel structure which governs the lengths, heights, diameter of bars, bending hock, and the composite of steel bars. In contrast, the concrete design mainly follows either the American standards or the British standard where the second one is considered as the main standard for concrete design. Unlike the steel design or the concrete one, the reinforced concrete has an international codes based on the needs of each region. In addition, the steel bars have specific standards shown in the following table .
Recommended end hook, all grades
Bar Size Finished bend diameter D, in 180 – deg hook
A or G J
# 3 2.25 5” 3”
#4 3 6” 4”
#5 3.75 7” 5”
#6 4.5 8” 6”
#7 5.25 10” 7”
#8 6 11” 8”
#9 9.5 1’-3” 11.75”
#10 10.75 1’-5” 1’-1.25”
#11 12 1’-7” 1’-2.75”
#14 18.25 2’-3” 1’-9.75”
#18 24 3’-0” 2’-4.5”
As you notice, from this table you can obtain mainly every thing about the dimensions of the steel bar by using specific equations the standard gives to you. For example, to find the diameter of steel bar #8 “pronounce as: Number eight” you follow the following equation to obtain the radius in feet: ( J and D have to be in Inches)
r (ft) = ( J - D )/24
Stability of the structure. Frankly, this point doesn’t depend on the design itself as it depends on the mind of the structural engineer because according to the usage of the structure the design is selected. However, comparing between the three types of design the most stable design is the steel-concrete because it combines the features of both steel and concrete designs.
Uses and selected design .In general, the steel design is selected to construct stores, small shops, and skyscrapers. Recently, there is some research to construct houses by native steel. On the other hand, the concrete design is selected to construct big structures such as villas, universities, power stations either electrical or nuclear one. However, the steel-concrete design is selected mainly for any type of structures, it is widely used when there is a huge lift – up force due to water if the building is constructed on unstable soil or even a huge wind force, almost more than 2KPa hurricane.
Environmental issues. We mean in this point that which design consumes less energy and causes less amount of pollution, and the best one is the steel design because due to the cover and insulation systems or foams, the steel would not get rust or converted into any other hazards. In contrast, the concrete or steel-concrete designs after the concrete gets its fully strength the amount of sulfur oxides will start to release and that would affect on the environment
Ahmed S. Dalloul
Civil Engineering Depart.